EEG or brain wave monitoring is a well established technique for evaluating brain function to assist in diagnosis of neurological disorders. Such monitoring has been most reliably performed on an inpatient basis such as in a hospital or similar facility. Monitoring in such a setting has many advantages. Hospitals and similar facilities have access to the large and expensive EEG machines typically needed for such monitoring. They also have facilities for caring for the patient on a long term basis, such monitoring typically being performed for a period on the order of ten days. Trained electrophysiologists are available in such facilities to secure electrodes to the patient, attach the electrodes to an EEG machine, and periodically observe the monitoring apparatus during the procedure to ensure that useful results are obtained. The ability in such a setting to shield the apparatus from stray signals which might interfere with the very small signals to be detected, and the restriction of patient movement to reduce movement-generated artifacts, contribute to the likelihood of obtaining useful results. Such inpatient monitoring is often the most desirable method for obtaining EEG data for diagnosis. However, it is a time consuming and expensive procedure, and the availability of inpatient facilities for such monitoring is often limited.
EEG monitoring is also performed on an outpatient basis, such as in a neurologist's office, hospital outpatient facilities, or the like. Outpatient EEG monitoring is typically performed for a short time, e.g. less than an hour, as a routine EEG study. Such routine studies may yield equivocal results or fail to detect brainwaves due to episodic events, but are useful to provide baseline data and as a relatively low cost screening procedure to determine whether a patient should undergo long term inpatient monitoring.
In order to provide some of the benefits of long-term monitoring without the inconvenience and expense of inpatient monitoring, various attempts have been made to develop systems for ambulatory EEG monitoring, i.e. monitoring using portable apparatus coupled to the patient, while the patient is not confined to a hospital or other medical facility. Such monitoring is highly desirable from a cost and convenience standpoint, in those situations where patient confinement during the monitoring procedure is not otherwise required. For instance, a patient may complain of episodic events such as "funny spells". A routine EEG study may provide normal results, and it will then be unclear whether this result is due to the fact that the patient has routine brain function or that no episodic events occurred during the routine EEG procedure. It would be highly desirable to obtain further EEG information without the expense, delay, and inconvenience of inpatient monitoring.
Existing ambulatory EEG systems to address these needs have been subject to a number of drawbacks limiting their use and usefulness. These drawbacks stem generally from the extremely large amounts of data which must be obtained for useful EEG monitoring and the difficulty in obtaining high quality data. The patient being monitored is not subject to the restrictions of an inpatient setting. To be useful, an ambulatory EEG system should have the capability of storing data occurring over a period on the order of day or more. During the monitoring period, it should have the capability of storing waveform data regarding at least several events, each of which may be several minutes in duration. It should further have the ability to store waveform data occurring immediately prior to an event. Such a monitoring system should also have the capability of simultaneously storing waveform data from a large number of channels during each event. One professional association in the field has issued guidelines specifying that at least 16 channels of waveform data should be obtained in inpatient EEG monitoring. However, because of the lack of adequate available ambulatory systems meeting these guidelines, no such guidelines have been issued for ambulatory EEG monitoring.
In order to accommodate the large amount of EEG data to be stored, prior art systems have generally relied upon magnetic tape recorders as the storage medium. One of the inventors of the present invention has developed such a system, which has been sold by Telefactor Corporation under the designation "A1-A2". The shortcomings of tape recorders for storing data generated in ambulatory patient monitoring is known, and is described for instance in U.S. Pat. No. 4,519,398 issued May 20, 1985 to Lisiecki et al. Generally, these shortcomings include size, weight, power consumption reliability, and quality of stored signals, which are typically stored as analog signals.
Various attempts have been made to limit the use of tape recorders in ambulatory physiological monitoring. For instance, one prior art ambulatory EEG system uses a combination of solid state memory and magnetic tape storage. This system responds to switch actuation, typically by the patient when an event begins, and records on magnetic tape the brain wave signals received for a predetermined time after switch actuation. Solid state memory is employed as a buffer for temporarily holding data. Incoming signals are stored in the memory configured as a first-in first-out device; when the event switch is actuated, the contents of the memory are latched to reflect the pre-actuation brain wave signals which may be relevant to the onset of the event. The latched data is then transferred to magnetic tape, so that the solid state memory is available to store pre-actuation data for the next event. All permanently stored data is stored on magnetic tape, and the solid state memory serves merely as a convenient means for temporarily capturing data occurring prior to switch actuation.
Lisiecki et al. U.S. Pat. No. 4,519,398 discloses a system for ambulatory monitoring of cardiac information, particularly heart rate and blood pressure information. The amount of data required to be stored in such monitoring is extremely limited. In order to store even this limited amount of data in a solid state memory, Lisiecki et al. require that the signals from the patient physiological sensors be highly processed prior to storage. Lisiecki et al do not store waveform data for the parameters being monitored; rather, numerical data representing heart rate and blood pressure computed from the waveform data relating to multiple channel EEG monitoring. Moreover, although Lisiecki et al. state that the disclosed system eliminates the use of a tape recorder, they contradictorily state that the equipment required to be used with the system of their invention includes a magnetic tape recorder, in column 2 line 23 through column 3 line 25.
Existing ambulatory EEG monitoring systems also have shortcomings in their mode of acquisition, analysis, and distribution of monitoring results. After a physician prescribes ambulatory EEG monitoring, the patient may be outfitted with monitoring equipment by a nurse or other office staff member who is not well trained in EEG. The patient goes home wearing the monitoring equipment, and data is recorded on magnetic tape over a period of a day or so. The equipment is then removed from the patient, and the tape is sent to an electroencephalographer for analysis of the captured data. This may include an initial high speed audio or video review of the tape by an EEG technician to attempt to locate portions of potential neurological significance. This initial review is performed to reduce the amount of data to be printed and reviewed by an electroencephalographer, but may result in significant events being overlooked. The waveform data on the identified portions of the tape is then printed, typically by a moving-pen type charter or plotter. The quality of waveforms produced by such devices is generally poor. The printed waveforms corresponding to identified potential events are then reviewed by an electroencephalographer, who prepares narrative report which is sent to the referring physician. The referring physician receives only the narrative report; waveform printouts on chart paper are difficult to store in patient files and are stored, if at all, only by the electroencephalographer. If the referring physician has any questions or requires additional information, he generally must call the electroencephalographer and attempt to satisfy his concerns by a telephone discussion without the benefit of waveform printouts.
Extensive information has been published regarding EEG monitoring in general and ambulatory EEG monitoring in particular. For further information regarding existing ambulatory EEG monitoring, including monitoring systems, procedures, clinical tests, indications, and limitations, reference may be made to "Ambulatory EED Monitoring", John S. Ebersole, Raven Press, 1989; "Clinical Usefullness of Ambulatory EEG", John S. Ebersole, Samuel L. Bridgers, Marshall J. Keilson, American Academy of Neurology 1990 Annual Meeting Seminar 267; and "Ambulatory Outpatient EEG Monitoring (A1-A2)", John R. Ives and Donald L. Schoner, American Academy of Neurology Annual Course #450 (April-May, 1990). These publications provide further background to the present invention; since much of their disclosures is pertinent to the present invention, they are incorporated herein by reference to the extent necessary to render this specification complete.